This invention relates generally to target cell screening for the ability to internalize ligands, and in particular, to identification of cells or tissues that bind specific ligands, internalize those ligands, and exhibit transgene expression.
Bacteriophage expressing a peptide on its surface has been used to identify protein binding domains, including antigenic determinants, antibodies that are specifically reactive, mutants with high affinity binding, identify novel ligands and substrate sites for enzymes. In its most common form, a peptide is expressed as a fusion protein with a capsid protein of a filamentous phage. This results in the display of the foreign protein on the surface of the phage particle. Libraries of phages are generated that express a multitude of foreign proteins. These libraries are bound to a substrate or cell that presents the binding partner of interest. This screening process is essentially an affinity purification. Bound phage are recovered, propagated, and the gene encoding the foreign protein may be isolated and characterized. This technology is commonly referred to as xe2x80x9cphage display.xe2x80x9d
Through a process called xe2x80x9cbiopanning,xe2x80x9d the specific phage carrying a peptide or protein that interacts with a protein or other moiety on a solid phase can be identified and isolated. However, in some applications, binding or binding affinity is not the sole critical parameter. For example, in gene therapy, a gene sequence needs to be introduced into a cell. In preferred methods, the gene sequence is targeted to particular cells by way of a ligand/cell surface receptor interaction. Thus, the ligand must not only bind to the cells but must also be internalized. A native ligand that is internalized, when used in a system for gene therapy may not be efficiently internalized. For example, both FGF2 and EGF are internalizing ligands; however, of these two ligands, FGF (or polypeptides reactive with the FGF receptor) is currently preferred as a gene targeting ligand.
Phage libraries can be screened for internalizing ligands by biopanning on live cells and rescuing internalized phage from the cells after stripping off externally bound phage (e.g., acid elution). However, this method may result in recovery of undesired phage that bind very tightly or are only partially internalized. Moreover, phage that are internalized and subjected to proteases lose infectivity and can not be recovered. Accordingly, current methodologies are inadequate to determine the usefulness of ligands for gene therapy.
Further, identification of target cells or tissues that are able to internalize ligands and express a transgene would readily allow one to identify specific target cells for known or putative ligands as well as allow one to identify ligands for specific cell or tissue types. However, current methods of target cell identification are hampered by the same difficulties, as noted above, with regard to screening for internalizing ligands. Accordingly, current methodologies are inadequate to determine which cell or tissue types are useful targets for ligand mediated gene therapy.
Thus, current screening methods are inadequate for identifying cell or tissue types that bind and internalize known or putative ligands. The present invention discloses a ligand display method that identifies target cells or tissue types that bind internalizing ligands as well as identifying the specific ligands that internalize in the target cell or tissue, and further provides other related advantages.
Within one aspect of the present invention, a method of identifying a target cell or tissue for internalizing ligands is presented, comprising: (a) contacting a library of ligand displaying genetic packages with a cell(s) or tissue(s), wherein each package carries a gene encoding a detectable product which is expressed upon internalization of the package; and (b) detecting product expressed by the cell(s) or tissue(s), and thereby identifying a target cell or tissue for internalizing ligands.
In another aspect, the invention provides a method of selecting an internalizing ligand for a selected target cell or tissue within a pool of target cells or tissues and identifying a target cell or tissue for the internalizing ligand, comprising: (a) contacting a library of ligand displaying genetic packages with a pool of cell(s) or tissue(s), wherein each package carries a gene encoding a detectable product which is expressed upon internalization of the package; (b) detecting product expressed by the cell(s) or tissue(s); and (c) recovering a nucleic acid molecule encoding an internalizing ligand from a selected set of cell(s) or tissue(s) within the pool expressing the product.
In yet another aspect, the invention provides a method of selecting an internalizing ligand for a selected target cell or tissue within a pool of target cells or tissues and identifying a target cell or tissue for the internalizing ligand, comprising: (a) contacting a library of ligand displaying genetic packages with a pool of cell(s) or tissue(s), wherein each package carries a gene encoding a detectable product which is expressed upon internalization of the package; (b) incubating the cell(s) or tissue(s) under selective conditions; and (c) recovering a nucleic acid molecule encoding an internalizing ligand from a selected set of cell(s) or tissue(s) within the pool which grow under the selective conditions; thereby selecting internalizing ligands and identifying a target cell or tissue for the internalizing ligand.
In yet another aspect, a method is provided for a high throughput method of identifying target cells or tissues for internalizing ligands, comprising: (a) contacting a library of ligand displaying genetic packages with cells or tissue in an array, wherein each package carries a gene encoding at least one detectable product which is expressed upon internalization of the package; and (b) detecting product(s) expressed by the cells or tissue in the array; thereby identifying target cells or tissues for internalizing ligands. In one embodiment, the array contains a variety of cell types. In another embodiment, the method further comprises step (c), wherein the library is a library of ligand displaying bacteriophages that is repeatedly divided into subset pools and screened using steps (a) and (b) until a specific bacteriophage expressing an internalizing ligand is identified.
In preferred embodiments, the ligand displaying genetic package comprises a bacteriophage. The bacteriophage are filamentous phage or lambdoid phage in other preferred embodiments. In some embodiments, the bacteriophage carries a genome vector.
In other embodiments, the library is a cDNA library, an antibody gene library, a random peptide gene library, a mutein library, or a pathogen coat/envelope gene library. In other preferred embodiments, the detectable product is selected from the group consisting of green fluorescent protein, xcex2-galactosidase, secreted alkaline phosphatase, chloramphenicol acetyltransferase, luciferase, human growth hormone and neomycin phosphotransferase.
In other embodiments, the cells may be isolated by flow cytometry, for example.
These and other aspects of the present invention will become evident upon reference to the following detailed description and attached drawings. In addition, various references are set forth below which describe in more detail certain procedures or compositions (e.g., plasmids, etc.), and are therefore incorporated herein by reference in their entirety.